Method for making an anisotropic conductive film pointed conductive inserts

ABSTRACT

The invention relates to a method for making an anisotropic conductive film with pointed conductive inserts.  
     The method comprises the etching of at least one pattern (C 1 , K 1 ) in a single crystal substrate ( 15 ) in order to form at least one cell ( 22, 26 ) with a bottom intended for drawing the contour of an end of an insert ( 23, 27 ). The drawing of the pattern is for having at least one protruding tip appear in the bottom of the cell during the etching of the pattern along the ( 100 ) crystallographic plane of the substrate with limiting ( 111 ) or ( 110 ) planes of the pattern. The invention is applied to microconnector technology.

TECHNICAL FIELD AND PRIOR ART

The present invention relates to a method for making an anisotropicconductive film with sharp conductive inserts.

In the field of microconnector technology, there are several largefamilies of techniques for connecting chips and integrated circuits toan interconnection substrate: microwiring, connection by tape automatedbonding (TAP), connection by beads (flip-chip technique) and theanisotropic conductive film (ACF) technique. According to themicrowiring technique, connection is achieved through gold or aluminumwires. The TAB connection uses an intermediate strip including a networkof metal conductors. According to the flip-chip technique, theinput/output pads are connected through brazes (fusible beads). The ACFtechnique applies conductive films consisting of metal particlesincorporated into an insulating film or of metal inserts included intoan insulating film. The intended electrical bonds between aninterconnection substrate and a chip are then established bythermocompression, by placing the conductive films between the substrateand the chip.

FIGS. 1A-1F, 2, 3A-3C illustrate a known method for making a conductivefilm with sharp inserts disclosed in French Patent No. 2,766,618.

A first step of the method consists of etching a substrate 1, forexample a silicon substrate. For this, a planar face 2 of a substrate 1with the (110) crystallographic plane is covered with a silicon nitrideor gold mask 3. The mask 3 is etched by a lithographical technique sothat the planar face 2 of the substrate appears through apertures 4 (cf.FIG. 1A). The exposed portions of the planar face 2 then receive achemical etching, for example by using KOH, along the (111)crystallographic planes. Cells 5 are thereby formed (cf. FIG. 1B). Whatremains of mask 3 is then removed and deposition of a conductivesacrificial layer 6 on the etched face of the substrate (cf. FIG. 1C) isperformed. The layer 6 may be made in copper (Cu), titanium (Ti), nickel(Ni), or tin/lead (SnPb). The thickness of the layer 6, for examplebetween 0.1 and 0.3 μm, conforms to the profile of the etched face. Apolymer layer 7, for example a polyimide layer with a thickness of 10μm, is deposited on the sacrificial layer 6. The polymer layer 7 isetched by photolithography in order to form holes 8 in the extension ofthe cells 5 (cf. FIG. 1D).

Metal inserts 9 are formed from the bottom of the cells 5 up to theupper face of the polymer layer 7 (cf. FIG. 1E), by electrolytic growth,using the sacrificial layer 6 as an electrode. The last step consists ofchemically etching the metal layer 6 in order to obtain detachment ofthe insulating film 7 provided with the conductive inserts 9 (cf. FIG.1F).

The etching of the silicon substrate 1 is carried out so that the cells5 are of a pyramidal shape with a square section. Accordingly, theinserts 9 are provided with tips 10. Moreover, the holes 8 have acircular cross-section with a smaller size than the cross-section of thecells 5 at the face 2 of the substrate. The inserts 9 are then embeddedinto the insulating film 7 as illustrated in FIG. 2.

A drawback of the method for making an anisotropic conductive filmdescribed above, is that it only allows the making of inserts providedwith a single tip. If the intention is to make inserts provided with twotips (one at each end of the insert), the making method needs to bechanged beyond the step which leads to the formation of a structure asillustrated in FIG. 1D. This change in the method is illustrated inFIGS. 3A-3C. A mask 11 is then positioned at a predetermined distance dabove the insulating film 7. The mask 11 is provided with holes 12positioned facing the holes 8 (cf. FIG. 3A). The metal for forming theinserts is then sprayed or evaporated through the holes 12 of the mask.The distance d which separates the mask 12 from the insulating film andthe diameter of the holes of the mask 12 are selected so as to give apointed shape 13 (cf. FIG. 3B) to the ends of the inserts located on theside of the mask. Subsequently, detachment of the insulating film 7 isperformed by chemical etching of the conducting layer 6, for examplewith hydrofluoric acid. The outcome of this is an anisotropic conductivefilm 7 provided with inserts 14 with a tip at each end (cf. FIG. 3C).

Advantageously, with the alternative method of the known art mentionedabove, the inserts may be made with two pointed ends. A drawback of thisalternative however lies in the fact that a mask provided with holesmust be placed above the film very accurately. The use of such a maskthen limits the pitch of the inserts to about 50 μm.

The invention does not have the above drawbacks.

DISCUSSION OF THE INVENTION

Indeed, the invention relates to a method for making an anisotropicconductive film with conductive inserts, the method comprising theetching of a least one pattern in a single crystal substrate in order toform at least one cell with a bottom for drawing the contour of a firstend of an insert. The drawing of the pattern is intended for having atleast one protruding tip and at least one recessed area appear in thebottom of the cell, during the etching of the pattern along at least onecrystallographic plane of the substrate with limiting crystallographicplanes.

By protrusion, a pointed area of the substrate is meant, pointingupwards as opposed to a recessed area of the substrate which pointsdownwards to the bottom of the substrate.

The inserts obtained according to the method of the invention aredissymmetrical. Thus, an insert formed from the cell at the end oppositeto its first end has at least one protruding tip and at least onerecessed area, the protruding portion and the recessed area facing arecessed area and a protruding tip of the first end of the insert,respectively.

According to a particular embodiment, the crystallographic plane alongwhich the pattern is etched, is the (100) plane and the limitingcrystallographic planes are the (111) and (110) planes.

Advantageously, with the making method according to the invention,conductive inserts with very small dimensions, spaced apart with a verysmall pitch (typically 1 to 2 μm inserts may be spaced apart by 4 to 5μm) may be obtained. Advantageously, the inserts may have several tipsat each end, thereby promoting electrical contact between the componentsto be assembled.

Advantageously, the method is simple and reproducible. Metal inserts arepreferentially made by electrolysis. With this method, the shape of theinserts is directly linked to the topology of the cell formed in thesubstrate. It is also possible to make the insert by spraying orevaporating metal.

The topology of the cell in which are formed the inserts, is obtained byetching patterns at the surface of a substrate. The layout of thepatterns is preferentially selected so as to provide electrolytic growthcapable of developing tips at both ends of the inserts.

The substrate consists of single crystal material for which wet etchingis anisotropic (i.e. for which the etching rate depends on the crystalplanes). For example silicon (Si) or silicon carbide (SiC) may bementioned.

The parameters to be defined for obtaining a cell topology according tothe invention are: the shape of the patterns, the orientation of thepatterns relatively to the directions of the crystallographic planes,and, in the case of several patterns, the mutual arrangement of thepatterns. A cell may be made, for example, from a group of simplepatterns, from a truncated square, from several groups of simplepatterns or even from several groups of truncated squares.

For example, a group of simple patterns may consist of at least foursimple patterns, for example four circles or four squares, specificallypositioned and orientated. A simple pattern is etched along the (100)crystallographic plane with limiting (111) or (110) planes. During theetching, the pattern is widened either because of the geometry of thepattern (for example, in the case of a circle) or by the orientation ofthe pattern relatively to the <110> direction of the crystal lattice(the case of deforming squares), or because of the overetchingphenomenon (etching under a mask).

The selected arrangement of simple patterns results in the widening ofthe patterns allowing them to join. When the patterns join, anisotropicwet etching uncovers new crystal planes other than the limiting (111)and (110) planes. Etching of the area surrounded by the simple patternsthen starts. As this area includes limiting (111) and (110) crystalplanes and non-limiting planes, a pointed topology is created.

Hence, the etching of a substrate mainly consists of two phases. A firstphase is a phase during which the patterns are etched independently ofeach other. The second phase (related to the shape and to thepositioning of the patterns) is a phase during which the etchings of thepatterns join and etching of the area surrounding the patterns starts.With this time lag between the first and second phases, a topology withtip(s) may be achieved in the cavities.

SHORT DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will become apparent fromthe description of a preferential embodiment made with reference to theappended figures, wherein:

FIGS. 1A-1F and 2 illustrate different steps of a method for making ananisotropic conductive film with pointed inserts according to the priorart;

FIGS. 3A-3C illustrate an alternative to the making method illustratedin FIGS. 1A-1F and 2;

FIGS. 4A-4F illustrate different steps of a first embodiment of themethod for making an anisotropic conductive film with pointed insertsaccording to the invention;

FIGS. 5A-5D, 6, 7, 8 and 9 illustrate examples of patterns for obtainingpointed inserts according to the method of the invention;

FIGS. 10A-10F illustrate different steps of a second embodiment of themethod for making an anisotropic conductive film according to theinvention;

FIGS. 11A-11B and 12A-12B illustrate examples of pointed inserts as wellas examples of the positioning of pointed inserts in an insulating filmaccording to the invention.

In all the figures, the same references designate the same components.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIGS. 4A-4F illustrate different steps of a first embodiment of themethod for making an anisotropic conductive film according to theinvention.

The first step of this method consists of etching a silicon substrate15, for example. For this, the planar face 16 of the substrate 15 with a(110) crystallographic plane, is covered with a mask 17 in siliconnitride, gold, copper, or any other material compatible with anisotropicwet etching. The mask 17 is etched with a lithographic technique, sothat the face 16 of the substrate 15 appears through the apertures 18(FIG. 4A). The exposed portions of the planar face 16 then receive achemical etching (for example by using KOH) along the (111)crystallographic planes. Cells 18 including tips 19 (cf. FIG. 4B) areobtained.

What remains of the mask 17 is then removed and deposition of asacrificial, for example conductive, layer 20 is performed on the etchedface 16 of the substrate 15 (cf. FIG. 4C). The layer 20 conforms to theprofile of the etched face 16. It may be made in Cu, Ti, Ni or SnPb. Itsthickness is between 0.1 and 3 μm for example.

A polymer layer 21 (for example a polyimide layer with a thickness of 10μm) is coated onto the metal layer 20 by a photolithographic technique,the layer 21 is etched in order to form circular holes 22 therein,aligned with the tips 19 of the substrate 15 (cf. FIG. 4D).

Metal inserts 23 are formed from the bottom of the cells up to the upperface of the polymer layer 21, in one step by electrolytic growth, usingthe metal layer 19 as an electrode, and by filling the holes 22 (cf.FIG. 4E). The metal which forms the metal inserts 3 may for example benickel or copper (FIG. 6E).

The last step consists of chemically etching the metal layer 20 in orderto obtain detachment of the insulating film 21 provided with the inserts23 (cf. FIG. 4F).

The holes 22 made in the insulated film 15 are of a circularcross-section. The cross-section of the holes 22 is less than thecross-section of the cells at the face 16 of the substrates 15, so thatthe inserts are embedded into the insulating film 15.

Wet etching may be completed with anisotropic etching (plasma etching)in order to enhance the height of the tips.

FIGS. 5A-5B, 6, 7, 8 and 9, illustrate examples of patterns forobtaining pointed inserts according to the method of the invention.

FIG. 5A illustrates a first pattern example for making tips in thesubstrate. The pattern consists of four circles C1, C2, C3, C4 mutuallypositioned so that their centres define a square. The axis passingthrough the centres of two circles which define a side of the squareforms a non-zero angle, for example equal to 45°, with the <110>direction of the crystal lattice. FIG. 5B illustrates the formation of acavity by anisotropic wet etching (for example a KOH-based etching) fromthe pattern illustrated in FIG. 5A. The four circles C1, C2, C3, C4, aretransformed into four squares K1, K2, K3, K4, respectively, the anglesof which join (cf. FIG. 5B). FIG. 5C illustrates the time course of theetching at the centre of the four circles with the occurrence of astar-shaped unetched area E having several slanted planes. Progressionof the etching leads to the formation of a tip P provided with edgesprotruding from the etched area (cf. FIG. 5D). The etching of the areassurrounding the central area is not illustrated in FIGS. 5C and 5D.

FIG. 6 illustrates another pattern example, consisting of four squares,the sides of which are not orientated along the <110> axis of thecrystal lattice.

Preferentially, the four squares K5, K6, K7, K8, are set up in order toform together a square pattern, each square with a side at an angle of45° relatively to the <110> direction of the crystal lattice. Theanisotropic wet etching of the four squares provides four squarecavities, the length of each square cavity being equal to the side ofthe initial square multiplied by {square root}{square root over (2)}.The etching of the squares leads to the formation of cavities, theangles of which join, and which form a protruding tip in their centre.

FIG. 7 illustrates a second pattern example formed on the basis of foursquares. The four squares K9, K10, K11, K12, are set up so as to formtogether a cross pattern, each square having two sides parallel to the<110> direction of the crystal lattice. Overetched areas S1, S2, S3, S4,surround the squares and enable the latter to join. The distance betweentwo squares depends on the depth of the desired overetching.

FIG. 8 illustrates a truncated square pattern made on the basis of twomasked areas M1, M2. Two parallel sides of the square are parallel tothe <110> crystallographic direction of the substrate. A first maskedarea M1 defines a square aperture, in which a second masked area M2 isplaced, also with a square shape, centered in the aperture defined bythe masked area M1. Etching is then performed between the masked areasM1 and M2 and completed with the formation of a tip centered in the M2area and protruding from the etching area.

FIG. 9 illustrates a pattern example for the formation of an insert withmultiple tips. The pattern is formed with four truncated squares. It isachieved on the basis of five masked areas. A first masked area M3defines a square aperture in which four other masked areas M4, M5, M6,M7, are placed. The four masked areas M4, M5, M6, M7, are positioned asa square. The etching of the unmasked substrate then generates a cavitywhich includes four tips protruding from the etched area.

FIGS. 10A-10F illustrate different steps of a second embodiment of themethod for making an anisotropic conductive film according to theinvention.

Up to the step for depositing a sacrificial layer, the method accordingto the second embodiment of the invention includes the same steps as themethod described earlier, i.e.: etching of a mask covering thesubstrate, chemical etching of the exposed substrate along determinedcrystallographic planes, removal of the mask and deposition of asacrificial layer.

Only the steps after the step for depositing the sacrificial layer willnow be described. A photoresist 24 is insolated through a mask in orderto form holes 26 in the extension of the tips 25 formed in the cells ofthe substrate (cf. FIG. 10A). Metal inserts 27 are made preferentiallyby electrolysis (cf. FIG. 10B) through the holes 26 of the resin.

Once the metal inserts are made, the resin is removed by dissolving itin a solvent (cf. FIG. 10C). An insulating film 28 is then deposited byknown methods of microelectronics onto the metal layer 20 and theinserts 27 (cf. FIG. 10D). With plasma etching of the insulating film28, it is possible to bring out the tips of the inserts (cf. FIG. 1E).The insulating film 28 is then detached (cf. FIG. 10F) for example withhydrofluoric acid.

FIGS. 11A-11B and 12A-12B illustrate exemplary embodiments of insertsaccording to the invention, as well as the positioning of these insertsin insulating film holes. FIGS. 11A-11B illustrate an insert with onetip and FIGS. 12A-12B illustrate an insert with four tips. Thecross-shaped inserts are placed in holes t of the insulating film.

There are several alternatives for some of the steps of the method ofthe invention. For example, the filling of the cells formed in thesubstrate may be achieved not only by electrolytic growth as mentionedabove, but also by spraying or evaporating metal. In both latter cases,the metal deposited on the surface of the photoresist must then beremoved. Several techniques are then possible, such as for example,mechanical lapping or mechano-chemical polishing.

According to the second embodiment of the invention, it is also possibleto first deposit the photoresist between the inserts and then theisolated film. The sacrificial layer is then etched and the photoresistis dissolved. It is also possible to dissolve the photoresist in orderto detach the anisotropic conductive film. With the latter alternative,the jutting out of the tips of the inserts relatively to the insulatingfilm may be enhanced.

By using silicon as a substrate, a perfectly defined and very sharp tipis obtained, providing a very high quality of electrical contact on analuminium pad.

When using a non-thermoplastic polymer for forming the insulating film,a slight spacing between the film and the chip to be connected, may bemaintained with the tips of the inserts, leaving the possibility ofusing an adhesive film on all the surfaces to be contacted and thereforeexcellent mechanical strength.

Regardless of its embodiment, the method for making an anisotropicconductive film with pointed inserts according to the invention enablesthe size of the inserts to be highly lowered, typically a diameter from1 to 2 μm for a pitch from 4 to 5 μm. This provides interconnection ofchips the inputs/outputs of which have a very small pitch.

Also, regardless of the embodiment, the etching step applied in themethod according to the invention may be completed by a further etchingstep for enhancing the heights of the tips. For example the furtheretching step may be purely anisotropic etching (plasma etching) orpurely isotropic etching (wet etching). This etching may be achievedbefore or after the first etching. The basic pattern may be of anyshape, as long as a less rapidly etched central area may be obtained.

The method according to the invention leads to the formation of atopology where the substrate has hollow areas with a very marked pointedshape. Advantageously, these hollow areas with a very marked pointedshape, allow very pointed metal inserts to be obtained during theelectrolysis, and this not only on the side where the insert has ahollow portion, but also on the other side. Indeed, the growth of metalinserts by electrolysis is enhanced by the presence of the strongtopology of the substrate. If the resin pattern is centered on a tip,the tip effect (faster growth related to current lines) enhances andmaintains the topology of the substrate. If the resin pattern issurrounded by four tips, a similar effect is obtained during theelectrolysis.

Advantageously, the ends of the conductive inserts are made in a hardmaterial (for example nickel) . This allows its ends to be able topierce through the oxide layer covering the pad to be connected. Theinserts may also be entirely made in this hard material. As analternative, only the extending-out portions of the inserts may be madein hard material.

The insulating film may be a thermoplastic polymer film or a multilayerfilm, the external layers of which are thermoplastic. With this, aself-adhesive function may be imparted to it during the assembly. In theopposite case, the insulating film must be provided with an adhesivelayer before assembly.

The anisotropic conductive film obtained by the method of the invention,enables a chip or an integrated circuit to be directly mounted on aninterconnection substrate, without it being necessary to specificallytreat the pads of the chip or integrated circuit.

1. A method for making an anisotropic conductive film with conductiveinserts, the method comprising the etching of at least one pattern (C1,K1) in a single crystal substrate (15) in order to form at least onecell (22, 26) with a bottom for drawing the contour of a first end of aninsert (23, 27), characterized in that the drawing of the pattern isintended for having at least one protruding tip and at least onerecessed area appear in the bottom of the cell, during the etching ofthe pattern along at least one crystallographic plane of the substratewith limiting crystallographic planes.
 2. The method according to claim1, characterized in that the crystallographic plane along which thepattern is etched, is the (100) plane and the limiting crystallographicplanes are the (111) and (110) planes.
 3. The method according to claim1, characterized in that a pattern is formed with a set of elementarypatterns separated from each other and positioned relatively to eachother so that, during the etching, the elementary patterns join, causingan area including limiting (111) and (110) planes and non-limitingplanes to appear between the patterns.
 4. The method according to claim3, characterized in that the elementary patterns are circles.
 5. Themethod according to claim 3, characterized in that the elementarypatterns are squares.
 6. The method according to claim 5, characterizedin that the squares are grouped parallel to each other so as to beinscribed in a square geometry, the sides of the squares not beingorientated along the <110> direction of the substrate.
 7. The methodaccording to claim 5, characterized in that the squares are groupedparallel to each other according to a cross-shaped geometry, each squarehaving two sides parallel to the <110> direction of the substrate, anoveretching area (S1, S2, S3, S4) surrounding the periphery of eachsquare.
 8. The method according to claim 1, characterized in that thepattern is formed with at least a truncated square, two parallel sidesof the square being parallel to the <110> direction of the substrate. 9.The method according to claim 1, characterized in that it comprises thedeposition of a sacrificial layer (20) onto the substrate, thesacrificial layer conforming to the profile of the cell.
 10. The methodaccording to claim 9, characterized in that it comprises the depositionof a polymer layer (21) onto the sacrificial layer (20) and in that thepolymer layer is etched in order to form circular holes (22) in theextension of the tips formed in the cell.
 11. The method according toclaim 10, characterized in that an insert is formed in a cell, from thebottom of the cell up to the level of an upper face of the polymerlayer.
 12. The method according to claim 11, characterized in that thesacrificial layer is etched in order to obtain detachment of the polymerlayer.
 13. The method according to claim 9, characterized in that aphotoresist is insolated through a mask for forming holes (26) in theextension of tips (25) formed in the substrate.
 14. The method accordingto claim 13, characterized in that an insert is formed in a hole formedin the photoresist.
 15. The method according to claim 14, characterizedin that the resin is removed by dissolving it in a solvent.
 16. Themethod according to claim 15, characterized in that an insulating filmis deposited on the sacrificial layer (20) and on the inserts (27). 17.The method according to claim 16, characterized in that plasma etchingof the insulating film causes the tips of the inserts to protrude. 18.The method according to claim 17, characterized in that the insulatingfilm provided with the inserts is detached from the sacrificial layer.19. The method according to claim 11, characterized in that the insertsare formed by electrolytic growth, by evaporation or spraying.
 20. Themethod according to claim 11, characterized in that the insert formedfrom the cell, has at the end opposite to its first end, at least oneprotruding tip and at least one recessed area, the protruding tip andthe recessed area facing a recessed area and a protruding tip of thefirst end of the insert, respectively.
 21. The method according to claim20, characterized in that the insert formed from the cell has, at itsfirst end, a protruding tip and at least two recessed areas.
 22. Themethod according to claim 20, characterized in that the insert formedfrom the cell has, at its first end, a recessed area and at least twoprotruding tips.
 23. The method according to claim 1, characterized inthat the inserts are in nickel or copper.
 24. The method according toclaim 1, characterized in that the substrate is in silicone or siliconcarbide.
 25. The method according to claim 1, characterized in that itcomprises a further etching step for increasing the tip height of theinsert.